Follow-up to https://golang.org/cl/21755.
This turned out to be a bit more than just a few nits
as originally expected in that CL.

1) The actual mantissa may be shorter than required for the
   given precision (because of trailing 0's): no need to
   allocate space for it (and transmit 0's). This can save
   a lot of space when the precision is high: E.g., for
   prec == 1000, 16 words or 128 bytes are required at the
   most, but if the actual number is short, it may be much
   less (for the test cases present, it's significantly less).

2) The actual mantissa may be longer than the number of
   words required for the given precision: make sure to
   not overflow when encoding in bytes.

3) Add more documentation.

4) Add more tests.

Change-Id: I9f40c408cfdd9183a8e81076d2f7d6c75e7a00e9
Reviewed-on: https://go-review.googlesource.com/22324Reviewed-by: Alan Donovan <adonovan@google.com>